1. Field of the Invention
This invention relates in general to a method and apparatus for mixing two gases in a predetermined proportion and in particular to a new and useful apparatus and method for continuously supplying a gas mixture from two separate gas supplies.
2. Description of the Prior Art
There is a known device for controlling the gas mixture of different kinds of gases in which each gas is expanded separately through a two-stage pressure reducer to a uniform pressure and then each gas is discharged through a respective adjustable metering nozzle controlling the proportion in the gas mixture into the mixer tube. In order to obtain a uniform final pressure after the pressure reducers, the pressure reducers are provided with an auxiliary control which receives control impulses from the uniform final pressure. The uniform final pressure may be maintained by means of an equalizing controller which acts on the control pressure of the pressure reducers. This has the effect that upon occurrence of a pressure difference in the gas lines between the pressure reducers and the metering nozzles at the side of the higher pressure, the control pressure drops and a smaller gas volume is delivered. At the other side the control pressure of the other pressure reducer increases so that a larger volume of gas is delivered. In this manner, an equalized low pressure is produced which is a supply condition for the metering nozzles. This known device has the disadvantage that even at an equalized supply pressure, the rate of flow through the metering nozzles depends on the pressure in the mixing tube. In addition the unequal inside diameters of the nozzles must be considered. In this respect the rate of flow depends on the pressure ratio. Thus the rate of flow and therefore the proportion of the mixture depends on two factors which are not easily controllable. Consequently such a device is unsuitable for apparatus in which the pressure differences can occur in the mixer tube. This disadvantage is of importance particularly in medical devices where for example if narcotics are admixed to a breathing gas there is a need for an exact proportion to be observed.
In another known method of mixing two gases in a predetermined proportion, the gases to be mixed are supplied through pressure governors which for adjusting the pressure ratio are coupled to each other. The outlet pressures of the pressure governors are arranged in an adjustable predetermined ratio to each other. Each gas flows at its own pressure into a respective chamber having a predetermined or adjustable volume. After the filling and interruption of the gas supply by a switch over valve, the gas flows into the gas outlet line through a further pair of pressure governors which are adjusted to the same pressure ratio as the first pressure governors. With such a method a gas mixture is produced which has a definite concentration since a definite gas quantity is measured off each gas to be mixed. These quantities after discharge mix with each other in accordance with the metered quantitative proportion. Concentration variations do not occur, not even at variations of the gas pressure in the connected operated device. To obtain this result however a relatively complicated arrangement is necessary, primarily because of the requirement for two pairs of pressure governors.